Conventional multi-blade fans used in homes or offices are disclosed in, e.g. Unexamined Japanese Patent Publication No. 2002-168194. One of these conventional multi-blade fans is described hereinafter with reference to FIG. 12 and FIG. 13.
FIG. 12 shows a general view of a conventional multi-blade fan, of which spirally-shaped housing 1 has bell-mouth orifice 2 on the upper side at the center. Housing 1 also has sucking inlet 3 and exhausting outlet 4. Housing 1 includes impeller 5 therein, which is driven by motor 6. Impeller 5 has a number of blades 9 supported by main plate 7 and lateral plate 8 at both the axial ends of respective blades. Air sucked from inlet 3 works as inflow stream 10 as the arrow mark in FIG. 12 shows and is guided to impeller 5.
FIG. 13 shows a sectional view cut along the direction vertical with respect to the rotary shaft of blades 9. A number of blades 9 in identical shape are annularly arranged at equal intervals. Each one of blades 9 shapes like as shown in FIG. 13, and has leading edge 11, trailing edge 12, and protrusion 14 on back face 13.
The air guided by orifice 2 flows like inflow stream 10 and exhausting stream 15 marked with the arrow marks. Separation vortices from back face 13 are suppressed by protrusion 14, thereby generating smaller vortices, which lower turbulent noise.
In the conventional multi-blade fan, however, blades 9 of impeller 5 still generate large vortices, so that the noise generated by impeller 5 is not yet satisfactorily suppressed, and needs to be lowered.